MOSFET and method for manufacturing MOSFET

ABSTRACT

A MOSFET includes a silicon carbide (SiC) substrate having a main surface having an off angle of not less than 50° and not more than 65° relative to a {0001} plane; a semiconductor layer formed on the main surface of the SiC substrate; and an insulating film formed in contact with a surface of the semiconductor layer. When the insulating film has a thickness of not less than 30 nm and not more than 46 nm, the threshold voltage thereof is not more than 2.3V. When the insulating film has a thickness of more than 46 nm and not more than 100 nm, the threshold voltage thereof is more than 2.3 V and not more than 4.9 V.

TECHNICAL FIELD

The present invention relates to a MOSFET and a method for manufacturing the MOSFET.

BACKGROUND ART

Conventionally, a semiconductor device using a silicon carbide (SiC) has been known (for example, WO01/018872 (hereinafter, referred to as Patent Document 1)). Patent Document 1 describes that a SiC substrate of 4H (Hexagonal) poly type having a plane orientation of almost {03-38} is used to form a Metal-Oxide-Semiconductor Field-effect Transistor (MOSFET). It is also described that in the MOSFET, a gate oxide film is formed by means of dry oxidation (thermal oxidation). Patent Document 1 describes that such a MOSFET achieves large channel mobility (approximately 100 cm²/Vs).

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: WO 01/018872

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

However, as a result of analysis and review, the present inventor has found that the channel mobility in the above-described MOSFET occasionally fails to be sufficiently large. When the channel mobility is not large, excellent characteristics of the semiconductor device thus employing SiC cannot be exhibited stably.

The present invention is made to solve the foregoing problem, and an object of the present invention is to provide a MOSFET having improved channel mobility and a method for manufacturing such a MOSFET.

Means for Solving the Problems

The present inventor has diligently diagnosed what renders channel mobility small, in order to realize large channel mobility in the MOSFET with good reproducibility as described above. As a result, the present invention has been accomplished. Specifically, the present inventor has found that the channel mobility becomes small due to a trap (hereinafter, also referred to as “interface state” or “interface state density”) existing in an interface between the gate insulating film and the SiC semiconductor film positioned below the gate insulating film. To realize a MOSFET with reduced influence of such an interface state, the present inventor has diligently studied and accordingly arrived at the present invention.

Specifically, a MOSFET according to one aspect of the present invention includes: a silicon carbide (SiC) substrate having a main surface having an off angle of not less than 50° and not more than 65° relative to a {0001} plane; a semiconductor layer formed on the main surface of the SiC substrate; and an insulating film formed in contact with a surface of the semiconductor layer and having a thickness of not less than 30 nm and not more than 46 nm, the MOSFET having a threshold voltage of not more than 2.3 V.

A method according to one aspect of the present invention for manufacturing a MOSFET includes the steps of: preparing a silicon carbide (SiC) substrate having a main surface having an off angle of not less than 50° and not more than 65° relative to a {0001} plane; forming a semiconductor layer on the main surface of the SiC substrate; and forming an insulating film in contact with a surface of the semiconductor layer and having a thickness of not less than 30 nm and not more than 46 nm, the MOSFET having a threshold voltage of 2.3 V or smaller.

A MOSFET according to another aspect of the present invention includes: a silicon carbide (SiC) substrate having a main surface having an off angle of not less than 50° and not more than 65° relative to a {0001} plane; a semiconductor layer formed on the main surface of the SiC substrate; and an insulating film formed in contact with a surface of the semiconductor layer and having a thickness of more than 46 nm and not more than 100 nm, the MOSFET having a threshold voltage of more than 2.3V and not more than 4.9 V.

A method according to another aspect of the present invention for manufacturing a MOSFET includes the steps of: preparing a silicon carbide (SiC) substrate having a main surface having an off angle of not less than 50° and not more than 65° relative to a {0001} plane; forming a semiconductor layer on the main surface of the SiC substrate; and forming an insulating film in contact with a surface of the semiconductor layer and having a thickness of more than 46 nm and not more than 100 nm, the MOSFET having a threshold voltage of more than 2.3 V and not more than 4.9V.

The present inventor has focused attention on the threshold voltage, which is associated with the interface state, and diligently studied a range of the threshold voltage so as to improve the mobility. As a result, the present inventor has found that by setting the threshold value voltage at not more than 2.3 V when the insulating layer has a thickness of not less than 30 nm and not more than 46 nm and setting the threshold voltage at more than 2.3 V and not more than 4.9 V when the insulating film has a thickness of more than 46 nm and not more than 100 nm, the interface state density can be reduced with good reproducibility in the vicinity of the interface between the insulating film and the semiconductor layer. In this way, most of carriers, which are to serve as an inversion channel layer, are prevented from being trapped in the interface state within the semiconductor layer at a region facing the insulating film. Therefore the channel mobility can be improved.

The lower limit of the off angle is set at 50° because it was observed that the carrier mobility is significantly increased as the off angle is increased in the course from a (01-14) plane in which the off angle is 43.3° to a (01-13) plane in which the off angle is 51.5° and because there is no natural plane in the range of the off angle between the (01-14) plane and the (01-13) plane.

Further, the upper limit of the off angle is 65° because it was observed that the carrier mobility is significantly decreased as the off angle is increased in the course of a (01-12) plane in which the off angle is 62.1° to a (01-10) plane in which the off angle is 90°, and because there is no natural plane in the range of the off angle between the (01-12) plane and the (01-10) plane.

Here, the term “threshold voltage” refers to a threshold voltage when the acceptor density is converted into 1×10¹⁶ cm⁻³.

Each of the MOSFETs in the above-described one and another aspects preferably has a sub-threshold slope of not more than 0.4 V.

The present inventor has further focused attention on the sub-threshold slope, which is associated with the interface state, and diligently studied a range of the sub-threshold slope so as to improve the mobility. As a result, the present inventor has found that by setting the sub-threshold slope at not more than 0.4 V/Decade, the interface state density can be reduced with good reproducibility near the interface between the insulating film and the semiconductor layer. In this way, most of carriers, which are to serve as an inversion channel layer, are prevented from being trapped in the interface state within the semiconductor layer at a region facing the insulating film. Therefore, the channel mobility can be improved more.

Each of the MOSFETs in the above-described one and another aspects preferably further includes a region including a nitrogen atom and interposed between the semiconductor layer and the insulating film.

In each of the MOSFETs in the above-described one and another aspects, a maximum value of nitrogen concentration is preferably 1×10²¹ cm⁻³ or greater in the region at a portion distant away by 10 nm or smaller from an interface between the semiconductor layer and the insulating film.

In the method for manufacturing the MOSFET in the above-described one and another aspects, the step of forming the insulating film preferably includes the steps of: forming the insulating film by thy oxidation; and thermally treating the insulating film using gas including a nitrogen atom as atmospheric gas.

The present inventor has found that the influence of the interface state is reduced by increasing the nitrogen atom concentration near the interface between the semiconductor layer and the insulating film. This achieved a MOSFET allowing for further improved channel mobility.

In each of the MOSFETs in the above-described one and another aspects, preferably, the semiconductor layer is formed of SiC. SiC, which has a large band gap, has a maximum dielectric breakdown electric field and a heat conductivity both larger than those of silicon (Si), and allows for carrier mobility as large as that in silicon. Also, in SiC, saturation drift velocity of electrons and withstand voltage are large. Accordingly, a MOSFET can be achieved which allows for high efficiency, high voltage, and large capacitance.

In each of the MOSFETs in the above-described one and another aspects, the main surface of the SiC substrate may have an off orientation falling within a range of ±5° of a <11-20> direction.

The <11-20> direction is a representative off orientation in the SiC substrate. Variation of the off orientation, which is caused by variation, etc., in a slicing process in a step of manufacturing the substrate, is set to be ±5°, thereby facilitating formation of an epitaxial layer on the SiC substrate, and the like. In this way, the MOSFET can be manufactured readily.

In each of the MOSFETs in the above-described one and another aspects, the main surface of the SiC substrate may have an off orientation falling within a range of ±5° of a <01-10> direction.

As with the <11-20> direction described above, the <01-10> direction is a representative off orientation in the SiC substrate. Variation of the off orientation, which is caused by variation, etc., in a slicing process in a step of manufacturing the substrate, is set to be ±5°, thereby facilitating formation of an epitaxial layer on the SiC substrate, and the like. In this way, the MOSFET can be manufactured readily.

In each of the MOSFETs in the above-described one and another aspects, the main surface of the SiC substrate can have an off angle of not less than −3° and not more than +5° relative to a plane orientation {03-38}.

In this way, the channel mobility can be further improved. Here, the off angle is thus set at not less than −3° and not more than +5° relative to the plane orientation {03-38} because particularly high channel mobility was obtained in this range as a result of inspecting a relation between the channel mobility and the off angle.

Here, the state in which “the off angle is not less than −3° and not more than +5° relative to the plane orientation {03-38}” refers to a state in which the orthogonal projection of a normal line of the main surface to a flat plane defined by the <0001> direction and the <01-10> direction serving as a reference for the off orientation forms an angle of not less than −3° and not more than +5° relative to a normal line of the {03-38} plane. The sign of a positive value corresponds to a case where the orthogonal projection approaches in parallel with the <01-10> direction whereas the sign of a negative value corresponds to a case where the orthogonal projection approaches in parallel with the <0001> direction.

It should be noted that the plane orientation of the main surface thereof is more preferably substantially {03-38} and the plane orientation of the main surface thereof is further preferably {03-38}. Here, the expression “the plane orientation of the main surface is substantially {03-38}” indicates that the plane orientation of the main surface of the substrate is included in a range of the off the angle in which the plane orientation of the substrate can be regarded as substantially {03-38} in consideration of precision of processing the substrate and the like. The range of the off angle in this case is a range in which the off angle is ±2° relative to {03-38}, for example. In this way, the above-described channel mobility can be improved further.

In each of the MOSFETs in the above-described one and another aspects, the main surface of the SiC substrate has an off angle of not less than −3° and not more than +5° relative to a (0-33-8) plane in the <01-10> direction.

As such, a structure is employed in which a semiconductor layer and an insulating film are formed on a surface close to the (0-33-8) plane, which is a plane particularly close to the C (carbon) plane in the {03-38} plane. In this way, the carrier mobility is improved significantly.

Here, in the present application, the (0001) plane of single-crystal silicon carbide of hexagonal crystal is defined as the silicon plane whereas the (000-1) plane is defined as the carbon plane. Meanwhile, the “off angle relative to the (0-33-8) plane in the <01-10> direction” refers to an angle formed by the orthogonal projection of a normal line of the main surface to a flat plane defined by the <000-1> direction and the <01-10> direction serving as a reference for the off orientation, and a normal line of the (0-33-8) plane. The sign of a positive value corresponds to a case where the orthogonal projection approaches in parallel with the <01-10> direction, whereas the sign of a negative value corresponds to a case where the orthogonal projection approaches in parallel with the <000-1> direction. Further, the expression “the main surface having an off angle of not less than −3° and not more than +5° relative to the (0-33-8) plane in the <01-10> direction” indicates that the main surface corresponds to a plane, at the carbon plane side, which satisfies the above-described conditions in the silicon carbide crystal. It should be noted that in the present application, the (0-33-8) plane includes an equivalent plane, at the carbon plane side, which is expressed in a different manner due to determination of an axis for defining a crystal plane, and does not includes a plane at the silicon plane side.

Effects of the Invention

As such, according to the MOSFET and the method for manufacturing the MOSFET in the present invention, the channel mobility can be improved by setting the threshold voltage at not more than 2.3 V when the insulating film has a thickness of not less than 30 nm and not more than 46 nm, and setting the threshold voltage at more than 2.3 V and not more than 4.9 V when the insulating film has a thickness of more than 46 nm and not more than 100 nm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view schematically showing a MOSFET of an embodiment of the present invention.

FIG. 2 illustrates a {03-38} plane in the embodiment of the present invention.

FIG. 3 is a flowchart showing a method for manufacturing the MOSFET in the embodiment of the present invention.

FIG. 4 is a schematic cross sectional view illustrating a step of the method for manufacturing the MOSFET of the embodiment of the present invention.

FIG. 5 is a schematic cross sectional view illustrating a step of the method for manufacturing the MOSFET of the embodiment of the present invention.

FIG. 6 is a schematic cross sectional view illustrating a step of the method for manufacturing the MOSFET of the embodiment of the present invention.

FIG. 7 is a schematic cross sectional view illustrating a step of the method for manufacturing the MOSFET of the embodiment of the present invention.

FIG. 8 shows a relation between a threshold voltage and interface state density in the second example.

FIG. 9 shows a relation between gate voltage and mobility in the third example.

FIG. 10 is a cross sectional view schematically showing an MOSFET fabricated in a fourth example.

FIG. 11 shows a relation between mobility and sub-threshold slope in a fourth examples.

FIG. 12 is a cross sectional view schematically showing a MOS capacitor fabricated in a fifth example.

FIG. 13 shows a relation between energy and interface state density in the fifth example.

FIG. 14 shows a relation between channel mobility and interface state density in the sixth example.

MODES FOR CARRYING OUT THE INVENTION

The following describes an embodiment of the present invention with reference to figures. It should be noted that in the below-mentioned figures, the same or corresponding portions are given the same reference characters and are not described repeatedly. It should be also noted that in the present specification, an individual orientation is represented by [ ], a group orientation is represented by < >, an individual plane is represented by ( ) and a group plane is represented by { }. In addition, crystallographically, a negative index is supposed to be indicated by putting a bar “−” above a numeral, but instead is indicated by putting a negative sign before the numeral in the present specification.

Referring to FIG. 1, a MOSFET 1 of one embodiment of the present invention will be described. MOSFET 1 of the present embodiment is a MOSFET of vertical type.

MOSFET 1 includes a substrate 2, a semiconductor layer 21, well regions 23, source regions 24, contact regions 25, an insulating film 26, a gate electrode 10, a source electrode 27, an interlayer insulating film 28, and a drain electrode 12.

Substrate 2 is, for example, an n⁺ SiC substrate. Substrate 2 has a main surface having an off angle of not less than 50° and not more than 65° relative to a {0001} plane. Preferably, the main surface thereof is a {03-38} plane. Here, as shown in FIG. 2, the {03-38} plane is a plane having a tilt of approximately 55°) (54.7° relative to the {0001} plane. In other words, the {03-38} plane is a plane having a tilt of approximately 35°) (35.3° relative to the direction of a <0001> axis.

It should be noted that the main surface of substrate 2 may have an off orientation falling within a range of ±5° of the <11-20> direction or a range of ±5° of the <01-10> direction. Further, the plane orientation of the main surface of substrate 2 may have an off angle of not less than −3° and not more than +5° relative to the plane orientation {03-38}. Furthermore, the main surface of substrate 2 may have an off angle of not less than −3° and not more than +5° relative to the (0-33-8) plane in the <01-10> direction. In these cases, channel mobility can be improved. In particular, by setting the plane orientation of the main surface of substrate 2 to (0-33-8), the channel mobility can be improved more.

On the main surface of substrate 2, a semiconductor layer 21 formed of, for example, an n type SiC is formed. Each of well regions 23 is positioned in a portion of the main surface of semiconductor layer 21 so as to form a pn junction with semiconductor layer 21. Well region 23 is p type SiC, for example. Each of source regions 24 is positioned in a portion of the main surface thereof within well region 23 so as to form a pn junction with well region 23. Source region 24 is SiC, for example. Each of contact regions 25 is positioned in a portion of the main surface thereof within well region 23 so as to form a pn junction with source region 24. Contact region 25 is SiC, for example.

Semiconductor layer 21 is of the same conductive type (n) as that of source region 24, and has an impurity concentration lower than that of source region 24.

Semiconductor layer 21 has a thickness of for example 10 μm. It is not particularly limited as to which one of the impurity concentration of semiconductor layer 21 and the impurity concentration of source region 24 is higher or lower. Source region 24 preferably has an impurity concentration higher than that of semiconductor layer 21, for example, has an impurity concentration of 1×10¹⁸ cm⁻³ to 1×10²⁰ cm⁻³. Examples of the n type impurity usable are: nitrogen (N), phosphorus (P), and the like.

Further, well region 23 is of a second conductive type (p) different from that of semiconductor layer 21. Examples of the p type impurity usable are: aluminum (Al), boron (B), and the like. Well region 23 has an impurity concentration of for example 5×10¹⁵ cm⁻³ to 5×10¹⁸ cm⁻³.

A region between source region 24 and semiconductor layer 21 in well region 23 serves as a channel of MOSFET 1. In the present embodiment, the conductive type thereof is determined to form an n channel, but the first and second conductive types may be determined in a manner opposite to that in the above-described case, so as to form a p channel.

Insulating film 26 insulates between semiconductor layer 21 and gate electrode 10, and is formed on and in contact with well region 23 between source region 24 and semiconductor layer 21. Insulating film 26 has a thickness of not less than 30 nm and not more than 100 nm.

Gate electrode 10 is formed on insulating film 26 to face at least well region 23 between source region 24 and semiconductor layer 21. It should be noted that gate electrode 10 may be also formed on another region as long as it is formed above well region 23 so as to face well region 23 between source region 24 and semiconductor layer 21.

On source region 24 and contact region 25, source electrode 27 is formed and is electrically connected to source region 24 and contact region 25. Source electrode 27 is electrically insulated from gate electrode 10 by insulating film 26.

Further, drain electrode 12 is formed on an opposite surface of substrate 2 to its surface making contact with semiconductor layer 21, and is thus electrically connected to substrate 2.

When insulating film 26 has a thickness of not less than 30 nm and not more than 46 nm, the threshold voltage of MOSFET 1 is not more than 2.3 V, and is preferably not less than 1.5 V and not more than 2.3 V. When insulating film 26 has a thickness of more than 46 nm and not more than 100 nm, the threshold voltage of the MOSFET is more than 2.3 V and not more than 4.9 V, and is preferably not less than 2.5 V and not more than 4.9 V. This leads to reduced interface state density, thereby achieving a large mobility.

Here, the threshold voltage will be described. The threshold voltage refers to a minimum gate voltage required to form a strongly inverted inversion channel layer in the channel. Threshold voltage V_(th) is expressed in the following Formula 1 with ∈₀ being the permittivity of vacuum, ∈_(SiC) being the permittivity of SiC, k being the Boltzmann's constant, T being the absolute temperature, N_(a) being the acceptor density, n_(i) being the intrinsic carrier density, C_(ox) being the insulating film capacity, q being elementary charge, work function difference being φ_(m)-φ_(s), and effective fixed charge being Q_(eff):

$\begin{matrix} {V_{th} = {\frac{\sqrt[2]{ɛ_{0}ɛ_{{Si}\; C}k\; T\; N_{a}{\ln\left( \frac{N_{a}}{n_{i}} \right)}}}{C_{OX}} + {\frac{2k\; T}{q}{\ln\left( \frac{N_{a}}{n_{i}} \right)}} + \left( {\phi_{m} - \phi_{s}} \right) + V_{Q_{eff}}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack \end{matrix}$

In Formula 1, V_(Qeff) is expressed in the following Formula 2 with d_(ox) being the thickness of the insulating film and ∈_(ox) being the permittivity of the insulating film:

$\begin{matrix} {V_{Q_{eff}} = {{- \frac{q\; Q_{eff}}{C_{OX}}} = {{- q}\; Q_{eff}\frac{d_{ox}}{ɛ_{0}ɛ_{ox}}}}} & \left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack \end{matrix}$

Further, MOSFET 1 preferably has a sub-threshold slope of 0.4 V/Decade or smaller. This leads to further reduced interface state density, thereby achieving a larger mobility.

Now, the sub-threshold slope will be described. The term “sub-threshold slope (also referred to as “sub-threshold swing”, “S value”, or the like)” indicates a gate voltage not more than a threshold voltage and required to increase current flowing between the source and the drain by one digit. The sub-threshold slope is expressed in the following Formula 3 with the gate voltage being V_(G) and the drain current being I_(D):

$\begin{matrix} {S \equiv {\ln\;{10 \cdot \frac{\mathbb{d}V_{G}}{\mathbb{d}\left( {\ln\; I_{D}} \right)}}}} & \left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack \end{matrix}$

Further, a region including nitrogen atoms is preferably formed at an interface between semiconductor layer 21 and insulating film 26. For example, a region distant away from the interface between semiconductor layer 21 and insulating film 26 by 10 nm or smaller preferably has a nitrogen concentration of 1×10²¹ cm⁻³ at maximum. In this case, mobility (channel mobility) can be at a sufficiently large value in a channel region having a channel length (region between well regions 23 in semiconductor layer 21).

This is considered to be attained due to the following reasons. That is, when insulating film 26 is formed through thermal oxidation or the like, a multiplicity of interface states are formed in the interface between insulating film 26 and semiconductor layer 21. This will result in extremely small channel mobility in the channel region, if nothing is done therefor. To counteract this problem, the nitrogen atoms are introduced into the region at the interface between insulating film 26 and semiconductor layer 21 as described above. In this way, the channel mobility can be improved while reducing the influences of the interface states.

The following describes a method for manufacturing MOSFET 1 in the present embodiment.

First, as shown in FIG. 3, a substrate preparing step (S10) is performed. In this step, a SiC substrate whose conductive type is n type is prepared as substrate 2. The SiC substrate thus prepared has a main surface having an off angle of not less than 50° and not more than 65° relative to a plane orientation {0001}, for example, having a plane orientation of (03-38) or (0-33-8). Such a substrate can be obtained by slicing an ingot having the (0001) plane as its main surface into substrate 2 so that the (03-38) plane or (0-33-8) plane thereof is exposed as the main surface, for example. In this step, for further improvement of the channel mobility in MOSFET 1 to be manufactured, it is particularly preferable to prepare substrate 2 having the (0-33-8) plane as its main surface. Further, as substrate 2, a substrate having a specific resistance of 0.02 Ωcm may be used, for example.

Then, a semiconductor layer forming step (S20) is performed. Specifically, as shown in FIG. 4, semiconductor layer 21 is formed on the main surface of substrate 2. Semiconductor layer 21 is formed of SiC whose conductive type is n type, and has a thickness of 10 μm, for example. Further, n type impurity in semiconductor layer 21 can have a concentration of 1×10¹⁶ cm⁻³.

Then, an injecting step (S30) is performed. Specifically, impurity (for example, Al) whose conductive type is p type is injected into semiconductor layer 21, using as a mask an oxide film formed by means of photolithography and etching. In this way, well regions 23 are formed as shown in FIG. 5. Thereafter, the oxide film thus used is removed and an oxide film having a new pattern is formed using photolithography and etching. Using this oxide film as a mask, n type conductive impurity (for example, P) is injected into predetermined regions to form source regions 24. In a similar way, conductive impurity whose conductive type is p type is injected to form contact regions 25. As a result, a structure shown in FIG. 5 is obtained.

After such an injecting step (S30), activation annealing treatment is performed. This activation annealing treatment can be performed under conditions that, for example, argon (Ar) gas is employed as atmospheric gas, heating temperature is in a range of 1700-1800° C., and heating time is 30 minutes. This activation annealing activates the impurity in the ion-injected region, and restores crystallinity.

Next, a gate insulating film forming step (S40) is performed. Insulating film 26 to be formed is not less than 30 nm and not more than 100 nm in thickness.

Specifically, as shown in FIG. 6, insulating film 26 is formed to cover semiconductor layer 21, well regions 23, source regions 24, and contact regions 25. The formation of insulating film 26 may be performed through dry oxidation (thermal oxidation), for example. This dry oxidation can be performed under conditions that, for example, heating temperature is 1200° C., heating time is 30 minutes, and the like.

Then, a nitrogen annealing step (S50) is performed. Specifically, gas including nitrogen (N) atoms, such as nitrogen monoxide (NO) gas or dinitrogen oxide (N₂O) gas, is used as atmospheric gas for heat treatment. The atmospheric gas is preferably nitrogen oxide.

The heat treatment can be performed under conditions that, for example, heating temperature is not less than 1100° C. and not more than 1300° C. and heating time is not less than 30 minutes and not more than 120 minutes. As a result, the nitrogen atoms can be introduced into the vicinity of the interface between insulating film 26 and each of semiconductor layer 21, well regions 23, source regions 24, and contact regions 25, each of which is positioned below insulating film 26.

After this nitrogen annealing step (S50), additional annealing may be performed using Ar gas, which is inert gas. Specifically, the annealing may be performed using the Ar gas as the atmospheric gas, under conditions that heating temperature is 1100° C. and heating time is 60 minutes.

In addition, after the nitrogen annealing step, surface cleaning may be performed, such as organic cleaning, acid cleaning, or RCA cleaning.

Next, an electrode forming step (S60) is performed. Specifically, on insulating film 26, a layer of high-concentration n type poly Si or the like, which is to be gate electrode 10, is formed using a CVD (chemical vacuum deposition) method. On this layer, a resist film having a pattern provided with an opening at a region other than the region to be gate electrode 10 is formed using the photolithography method. Using this resist film as a mask, the layer's portion exposed from the pattern is removed by means of RIE (Reactive Ion Etching) or the like. In this way, gate electrode 10 can be formed as shown in FIG. 7.

Then, an insulating film formed of SiO₂ or the like, which is to be interlayer insulating film 28, is formed using the CVD method so as to cover gate electrode 10. For example, silicon oxide (SiO₂) or silicon nitride (Si₃N₄) may be deposited using the CVD method or a plasma CVD method. For example, SiO₂ may be deposited using the plasma CVD method thereon by for example 1 μm, under conditions that a raw material gas of tetraethoxysilane (TEOS) and oxygen (O₂) is used and heating temperature is 350° C. On the insulating film, a resist film having a pattern provided with an opening at a region other than the region to be interlayer insulating film 28 is formed using the photolithography method. Using the resist film as a mask, the insulating film's portion exposed from the pattern is removed using the RIE. In this way, interlayer insulating film 28 having openings can be formed as shown in FIG. 7.

Next, on interlayer insulating film 28, a resist film is formed using the photolithography method. The resist film has a pattern for exposing a portion of each source region 24 and each contact region 25. On the pattern and the resist, a conductive film of Ni or the like is formed. Thereafter, by removing (lifting off) the resist, a portion of each source electrode 27 can be formed in contact with source region 24 and contact region 25 each of which is exposed from insulating film 26 and interlayer insulating film 28. Further, drain electrode 12 is formed on the back-side surface of substrate 2. For drain electrode 12, nickel (Ni) can be used, for example. After forming source electrode 27 and drain electrode 12, heat treatment for alloying is performed, for example. In this way, a portion of source electrode 27 and drain electrode 12 can be formed as shown in FIG. 7.

Then, on the formed portion of source electrode 27, upper source electrode 27 is formed. Upper source electrode 27 can be formed by means of, for example, lift-off, etching, or the like. In this way, MOSFET 1 shown in FIG. 1 can be manufactured.

As described above, MOSFET 1 of the present embodiment includes: SiC substrate 2 having a main surface having an off angle of not less than 50° and not more than 65° relative to the {0001} plane, preferably, having the {03-38} plane; semiconductor layer 21 formed on SiC substrate 2; and insulating film 26 formed in contact with the surface of semiconductor layer 21. When insulating film 26 has a thickness of not less than 30 nm and not more than 46 nm, the threshold voltage is not more than 2.3V. When insulating film 26 has a thickness of more than 46 nm and not more than 100 nm, the threshold voltage is more than 2.3 V and not more than 4.9 V.

Meanwhile, the method for manufacturing MOSFET 1 in the present embodiment includes: the substrate preparing step (S10) of preparing SiC substrate 2 having a main surface having an off angle of not less than 50° and not more than 65° relative to the {0001} plane, preferably, having a {03-38} plane as the main surface; the semiconductor layer forming step (S20) of forming semiconductor layer 21 on SiC substrate 2; and the gate insulating film forming step (S40) of forming insulating film 26 in contact with the surface of semiconductor layer 21. When insulating film 26 has a thickness of not less than 30 nm and not more than 46 nm, the threshold voltage is not more than 2.3 V. When insulating film 26 has a thickness of more than 46 nm and not more than 100 nm, the threshold voltage is more than 2.3 V and not more than 4.9 V.

The present inventor has found that the interface state density can be effectively reduced in the vicinity of the interface between insulating film 26 and semiconductor layer 21 by setting the threshold voltage at not more than 2.3V when insulating film 26 has a thickness of not less than 30 nm and not more than 46 nm, and setting the threshold voltage at more than 2.3 V and not more than 4.9 V when insulating film 26 has a thickness of more than 46 nm and not more than 100 nm. This restrains most of carriers, which are to serve as an inversion channel layer, from being trapped in the interface states at the region facing insulating film 26 in semiconductor layer 21. This further prevents trapped carriers from behaving as fixed charges. Accordingly, applied voltage (threshold voltage) to the gate electrode can be maintained to be small, and most of the carriers contribute to a current between the source and the drain. Thus, MOSFET 1 allowing for improved channel mobility is obtained. As such, in the present embodiment, a large channel mobility can be realized with good reproducibility, thus allowing the excellent characteristics of MOSFET 1 to be exhibited stably.

FIRST EXAMPLE

In the present example, a relation between the thickness of the insulating film and the threshold voltage was examined.

The Present Invention's Examples 1-7

For each of the present invention's examples 1-7, the threshold voltage was simulated when insulating film 26 had a thickness of not less than 30 nm and not more than 100 nm in the structure of the MOSFET shown in FIG. 1 and fabricated under the below-described conditions. It should be noted that in each of the present invention's examples 1-7, the nitrogen annealing step (S50) was performed in manufacturing the MOSFET. Further, the main surface of the SiC substrate corresponded to the (03-38) plane. The acceptor density was 1×10¹⁶ cm⁻³. The gate electrode was poly Si. Fixed charge density Q_(eff) of electrons trapped in the interface states was −5×10¹¹ cm⁻². The channel direction (direction in which the drain current flows) was the <11-20> direction. The result is shown in Table 1 below.

Comparative Example 1

In a comparative example 1, in the structure of the MOSFET shown in FIG. 1, a SiC substrate was used which had a main surface having an off angle of 8° relative to the (0001) plane. Hence, in comparative example 1, fixed charge density Q_(eff) of electrons trapped in the interface states was −2×10¹² cm⁻². The result is shown in Table 1 below.

TABLE 1 (03-38) NO Thickness of (03-38) NO Threshold Threshold annealing not Threshold insulating film annealing performed voltage (0001) 8° off voltage performed voltage 30 (nm) The present 1.5 (V) — — — — invention's example 1 40 (nm) The present 2.0 (V) — — — — invention's example 2 46 (nm) The present 2.3 (V) Comparative 5.5 (V) Comparative 2.8 (V) invention's example 3 example 1 example 2 50 (nm) The present 2.5 (V) — — — — invention's example 4 60 (nm) The present 3.0 (V) — — — — invention's example 5 80 (nm) The present 4.0 (V) — — — — invention's example 6 100 (nm)  The present 4.9 (V) — — — — invention's example 7

Comparative Example 2

A comparative example 2 corresponded to the fourth example of Patent Document 1. Specifically, a MOSFET of comparative example 2 was a MOSFET in which the insulating film had a thickness of 46 nm and the channel direction was <11-20>.

Specifically, it is described that in the MOSFET of the fourth example of Patent Document 1, the thickness of the insulating film was 35-46 nm and the acceptor density was 5×10¹⁵ cm⁻³, In Patent Document 1, the nitrogen annealing step (S50) was not performed. In the MOSFET of Patent Document 1, with the acceptor density converted into 1×10¹⁶ cm⁻³, fixed charge density Q_(eff) of the electrons trapped in the interface states can be estimated as −7.5×10¹¹ cm⁻². In view of this, although Patent Document 1 illustrates a threshold voltage of 2.3-2.5V in FIG. 11, the threshold voltage was converted so as to correspond to the acceptor density thus converted into 1×10¹⁶ cm⁻³. The result is shown in Table 2 below.

TABLE 2 Thickness of Threshold voltage Threshold voltage insulating when acceptor density was when acceptor density was film 5 × 10¹⁵ cm⁻³ 1 × 10¹⁶ cm⁻³ 46 (nm) 2.4 (V) 2.8 (V) 40 (nm) 2.3 (V) 2.7 (V) 35 (nm) 2.3 (V) 2.6 (V)

As such, the MOSFET in which the insulating film had a thickness of 46 nm and the channel direction was <11-20> was regarded as comparative example 2.

It is apparent also from Formula 1 described above that the threshold voltage is increased as the acceptor density is increased.

Evaluation Result

The present invention's examples 3 and comparative example 1 were compared in which the respective thicknesses of the insulating films are the same. As shown in Table 1, the threshold voltage in the present invention's examples 3 was reduced as compared with the threshold voltage of comparative example 1.

Further, in the present invention's example 3 and comparative example 2 in each of which the insulating film had a thickness of 46 nm, the threshold voltages when the acceptor density was converted into 1×10¹⁶ cm⁻³ were compared. The threshold value of the present invention's examples 3 was reduced as compared with that of comparative example 2. Further, as shown in Table 2, each of all the MOSFETs disclosed in Patent Document 1 when the acceptor density was converted into 1×10¹⁶ cm⁻³ had a threshold voltage larger than the threshold voltage of the present invention's examples 3 having the insulating film whose thickness is equal to or larger than that of Patent Document 1. Further, a MOSFET of Patent Document 1 which has a channel direction of <11-20>, which is the same as the channel direction of the present invention's examples 3, had a threshold voltage of 2.4 V when the acceptor density was not converted into 1×10¹⁶ cm⁻³ (when the acceptor density was 5×10¹⁵ cm⁻³). For this reason, when the channel directions of the present invention's examples 3 and Patent Document 1 were the same, the threshold voltage in the present invention's example 3 was reduced as compared with the threshold voltage in Patent Document 1. This confirmed that the MOSFET of the present invention had a reduced threshold voltage as compared with the threshold voltage of Patent Document 1.

As described above, it was confirmed that a MOSFET can be realized in which the threshold voltage can be not more than 2.3 V when the insulating film has a thickness of not less than 30 nm and not more than 46 nm and the threshold voltage can be more than 2.3 V and not more than 4.9 V when the insulating film has a thickness of more than 46 nm and not more than 100 nm. In order to realize such a MOSFET, it was confirmed effective to thermally treat insulating film 26 using gas including nitrogen atoms as atmospheric gas (nitrogen annealing step (S50)).

SECOND EXAMPLE

In the present example, a relation between the thickness of the insulating film and the threshold voltage was examined. Also examined was an effect of achieving reduced interface state density in the MOSFET by reducing the threshold voltage.

Interface state densities were simulated for the MOSFET of the present invention's examples 3 and the MOSFET of comparative example 2. The result is shown in FIG. 8.

As shown in FIG. 8, it was found that as the threshold voltage is lower, the interface state density can be reduced.

In particular, in FIG. 8, it was found that the interface state density can be reduced significantly to 5×10¹¹ cm⁻² eV⁻¹ or smaller by setting the threshold voltage at not more than 2.3 V when the acceptor density is converted to 1×10¹⁶ cm⁻³ as illustrated in the present invention's examples 3 and comparative example 2 in each of which the insulating film had a thickness of 46 nm or smaller.

It was confirmed that the interface state density can be effectively reduced as described above by thermally treating the insulating film using gas including nitrogen atoms as atmospheric gas (nitrogen annealing step (S50)).

It is considered that such a reduced interface state density provides the following effect. That is, most of carriers, which are to serve as an inversion channel layer, are prevented from being trapped in the interface state within the semiconductor layer at a region facing the insulating film. Accordingly, the channel mobility of the MOSFET can be improved with good reproducibility.

Further, in the present example, the threshold voltage and the interface state density were examined in the case where the insulating film had a thickness of 46 nm or smaller, but the same is applied also in the case where the insulating film had a thickness of more than 46 nm. In other words, when the threshold voltage is low as in the present invention, the interface state density can be effectively reduced.

Here, in the present example, the SiC substrate having the (03-38) plane as its main surface was utilized as substrate 2 of the present invention's examples 3. However, the present inventor has found that a further lower threshold voltage can be realized by using a SiC substrate having a (0-33-8) plane as its main surface, thus achieving further reduced interface state density effectively.

THIRD EXAMPLE

In the present example, the effect of improving mobility in a MOSFET by reducing the threshold voltage was examined.

The Present Invention's Example 8

In the present invention's example 8, a MOSFET was prepared in which the insulating film had a thickness of 37 nm, had a threshold voltage of 1.8 V, and had a <11-20> channel direction. It should be noted that in the present invention's example 8, the nitrogen annealing step (S50) was performed in fabricating the MOSFET. The MOSFET was fed with a gate voltage and the channel mobility was measured. The result is shown in FIG. 9.

As shown in FIG. 9, the MOSFET of the present invention's example 8 had a channel mobility of 100 cm²/Vs. In the fourth example of Patent Document 1, the mobility was 96 cm²/Vs at maximum when the channel direction was the same as that in the present invention's example 8 (<11-20>). From this, it was found that the channel mobility of the MOSFET of the present invention's example 8 was improved as compared with the channel mobility of the MOSFET of Patent Document 1.

As such, according to the present example, it was confirmed that the channel mobility can be improved by reducing the threshold voltage.

FOURTH EXAMPLE

In the present example, the effect of improving mobility in a MOSFET by setting the sub-threshold slope thereof at not more than 0.4 V/Decade was examined.

The Present Invention's Examples 9, 10

As each of the MOSFETs of the present invention's examples 9 and 10, a MOSFET 3 of lateral type basically as shown in FIG. 10 was manufactured.

Specifically, first, in the substrate preparing step (S10), as substrate 2, a 4H—SiC substrate was prepared which had the (03-38) plane as the main surface thereof.

Next, in the semiconductor layer forming step (S20), as semiconductor layer 31, a p type SiC layer was formed which had a thickness of approximately 0.8 μm and had an impurity concentration of 1×10¹⁶ cm⁻³. The main surface of the p type SiC layer corresponded to the (03-38) plane.

Next, in the injecting step (S30), SiO₂ was used as the mask material. Using P as n type impurity, source regions 24 and drain regions 29 were formed to have an impurity concentration of 1×10¹⁹ cm⁻³. Meanwhile, using Al as p type impurity, contact regions 25 were formed to have an impurity concentration of 1×10¹⁹ cm⁻³.

After the injecting step (S30), activation annealing treatment was performed. The conditions of the activation annealing treatment were: Ar gas was used as an atmospheric gas, heating temperature was 1700-1800° C., and heating time was 30 minutes.

Then, in the gate insulating film forming step (S40), a gate oxide film was formed as insulating film 26 by means of dry oxidation, under conditions that heating temperature was 1200° C., and heating time was 30 minutes in the present invention's example 9 and 45 minutes in the present invention's example 10. In addition, surface cleaning was performed.

Then, in the nitrogen annealing step (S50), in an atmosphere including NO, heat treatment was performed under conditions that heating temperature was 1100° C. in the present invention's example 9 and 1200° C. in the present invention's example 10 and heating time was 120 minutes.

Next, in the electrode forming step (S60), gate electrode 10 formed of poly Si, source electrode 27 formed of Ni, and drain electrode 12 formed of Ni were formed.

By performing the steps (S10-S60), MOSFETs 3 of the present invention's examples 9 and 10 were manufactured.

Comparative Example 3

A MOSFET of a comparative example 3 was manufactured in basically the same way as the MOSFET of the present invention's example 9, but was different therefrom in conditions in that the main surface of the substrate was the (0001) plane, the nitrogen annealing step (S50) was not performed, and heating temperature was 1300° C. and heating time was 20 minutes in the gate insulating film forming step (S40).

Comparative Example 4

A MOSFET of a comparative example 4 was manufactured in basically the same way as the MOSFET of the present invention's example 9, but was different in that the main surface of the substrate was the (0001) plane, heating temperature was 1300° C. and heating time was 30 minutes in the gate insulating film forming step (S40), and heating temperature was 1300° C. and heating time was 60 minutes in the nitrogen annealing step (S50).

Comparative Example 5

A MOSFET of a comparative example 5 was manufactured in basically the same way as the MOSFET of the present invention's example 9, but was different therefrom in that the main surface of the substrate was the (0001) plane, heating temperature was 1300° C. and heating time was 30 minutes in the gate insulating film forming step (S40), and heating temperature was 1200° C. and heating time was 60 minutes in the nitrogen annealing step (S50).

Measuring Method

The mobility and sub-threshold slope of each of the MOSFETs of the present invention's examples 9 and 10 and comparative examples 3-5 were measured.

Specifically, for the mobility thereof, a source-drain current IDS was measured while applying a gate voltage V_(G) with a source-drain voltage V_(DS)=0.1 V (gate voltage dependency was measured). Then, the maximum value of the mobility relative to the gate voltage was determined as follows:

Channel mobility μ=g_(m)×(L×d)/(W×∈×V_(DS)), where g_(m)=(δI_(DS))/(δV_(G)), L indicates a gate length, d indicates the thickness of the oxide film, W indicates a gate width, and c indicates the permittivity of the oxide film.

The sub-threshold slope was measured as follows. While applying gate voltage V_(G) with source-drain voltage V_(DS)=0.1V, the sub-threshold slope was measured using Formula 3 in a range in which gate voltage V_(G) was equal to or smaller than the threshold voltage at a straight line region in a semilogarithmic plot of source-drain current I_(DS) relative to the gate voltage. The result thereof is shown in FIG. 11.

As shown in FIG. 11, the MOSFETs of the present invention's examples 9 and 10, each of which had a sub-threshold slope of 0.4, achieved a high mobility of not less than 74 (cm²/Vs) and not more than 92 (cm²/Vs). On the other hand, the MOSFETs of comparative examples 3-5, each of which has a sub-threshold slope of 0.9-1.0, had a low mobility of not less than 2.5 (cm²/Vs) and not more than 20 (cm²/Vs).

As such, according to the present example, it has been found that the mobility can be improved by setting the sub-threshold slope at not more than 0.4 V/Decade.

In the present example, the SiC substrate having the (03-38) plane as its main surface was employed as substrate 2 of the present invention's example 9 and 10, but the present inventor has found that the mobility can be further improved when using a SiC substrate having the (0-33-8) plane as its main surface.

FIFTH EXAMPLE

In the present example, the effect of achieving reduced interface state density by setting the sub-threshold slope at not more than 0.4 V/Decade was examined.

Generally, the interface state density is determined from the sub-threshold slope (S value) as follows. In Formula 4 described below, a theoretical S value obtained upon interface state density D₁=0 can be determined using a known insulating film capacity C_(ox), and a depletion layer capacity C_(a) upon strong inversion (calculated from the maximum width of the depletion layer upon strong inversion, for example).

$\begin{matrix} {S \equiv {\ln\; 10 \times \frac{k\; T}{q}\left( {1 + \frac{C_{d} + C_{it}}{C_{OX}}} \right)}} & \left\lbrack {{Formula}\mspace{14mu} 4} \right\rbrack \end{matrix}$

In Formula 4, k indicates a Boltzmann's constant, T indicates an absolute temperature, C_(d) indicates the depletion layer capacity upon strong inversion, C_(it) indicates the interface state capacity (C_(it)=qD_(it)), and C_(ox) indicates an oxide film capacity.

By comparing the S values in Formula 4 and the fourth example, interface state capacity C_(it) can be calculated to derive interface state density D_(it). However, it was found that D_(it) determined in accordance with Formula 4 is not so precise. In view of this, in the present example, a MOS capacitor 30 shown in FIG. 12 was fabricated as described below. In accordance with the capacitance/voltage characteristics thereof, interface state density D_(it) was determined with improved precision and was examined.

The Present Invention's Example 11

Specifically, first, as substrate 2, a substrate similar to that in the substrate preparing step (S10) of the present invention's example 9 was used.

Next, a semiconductor layer 21 similar to that in the semiconductor layer forming step (S20) of the present invention's example 9 was formed on substrate 2.

Then, an insulating film 26 similar to that in the gate insulating film forming step (S40) of the present invention's example 9 was formed on semiconductor layer 21.

Then, the nitrogen annealing step (S50) was performed in a manner similar to that in the present invention's example 9, except that heating temperature was 1100° C. and heating time was 60 minutes.

Then, on insulating film 26, a gate electrode 10 similar to that in the electrode forming step (S60) of the present invention's example 9 was formed. Further, as back-side contact electrode 18, Ni was formed.

In this way, the MOS capacitor of the present invention's example 11 was manufactured.

Comparative Example 6

A MOS capacitor of a comparative example 6 was manufactured in basically the same way as the MOS capacitor of the present invention's example 11, but was different therefrom in that heating temperature was 1200° C. and heating time was 30 minutes in the gate insulating film forming step (S40) and the nitrogen annealing step (S50) was not performed.

Comparative Example 7

A MOS capacitor of a comparative example 7 was manufactured in basically the same way as the MOS capacitor of the present invention's example 11, but was different therefrom in that the main surface of the substrate was the (0001) plane and heating temperature was 1300° C. and heating time was 60 minutes in the nitrogen annealing step (S50).

Measuring Method

Energy and interface state density in each of the MOS capacitors of the present invention's example 11 and comparative examples 6 and 7 were measured. It should be noted that the energy herein refers to energy in a band gap with respect to the bottom of the conduction band at the semiconductor layer side of the MOS interface (interface between semiconductor layer 21 and insulating film 26).

The interface state density was measured in accordance with capacitance C/voltage V characteristics by means of a High-Low method. The result is shown in FIG. 13.

As shown in FIG. 13, in the MOS capacitor of the present invention's example 11, the interface states at the MOS interface were low. From this fact, it was found that insulating film 26 was thermally treated using gas including nitrogen atoms as the atmospheric gas, thereby achieving reduced interface state density.

Further, when manufacturing a MOSFET under the conditions for the MOS capacitor of the present invention's example 11, the MOSFET had a sub-threshold slope of not more than 0.4. As such, it was found that the interface state density can be reduced by setting the sub-threshold slope at not more than 0.4.

It is considered that such a reduced interface state density provides the following effect. That is, inversion electrons can be reduced which do not contribute to the current between the source and the drain and are trapped in the interface states. This can reduce an applied gate voltage, i.e., threshold voltage, required to form inversion channel electrons necessary to let a current flow sufficiently between the source and the drain. Because the interface state density can be reduced by setting the sub-threshold slope at not more than 0.4 as such, it is considered that the mobility therein can be improved.

On the other hand, in the MOS capacitors of comparative examples 6 and 7, the interface states at the MOS interface were high. When manufacturing a MOSFET under the conditions for the MOS capacitors of comparative examples 6 and 7, the MOSFET had a sub-threshold slope of more than 0.4. Accordingly, it is considered that the threshold voltage is large because when inversion channel electrons are trapped in the MOS interface, the mobility in the MOSFET is decreased and they accordingly behave as negative fixed charges. In Patent Document 1 described above, the nitrogen annealing step (S50) is not performed as in comparative example 6. Hence, it is considered that in Patent Document 1, the interface state density is as large as that in comparative example 6. Accordingly, it can be said that high mobility is hardly achieved with good reproducibility in the MOSFET of Patent Document 1.

As described above, according to the present example, it has been found that by setting the sub-threshold slope at not more than 0.4 V/Decade, the interface state density can be reduced to improve the mobility. In addition, it is considered that such a large channel mobility can be realized with good reproducibility.

SIXTH EXAMPLE

In the present example, an effect of achieving improved mobility by reducing the interface state density was examined.

MOS capacitors of the present invention's example 9 and comparative examples 3 and 5 described in the fourth example were prepared. In each of the MOS capacitors of the present invention's examples 9, comparative example 9, and comparative example 11, the acceptor density was 1×10¹⁶ cm⁻³ and the insulating film had a thickness of 40 nm.

Each of the MOSFETs of the present invention's example 9, comparative example 3, and comparative example 5 is fed with a gate voltage and channel mobility thereof was measured. Further, interface state densities were found from MOS capacitors fabricated under the same conditions as those for comparative example 3 and comparative example 5. As a result, a relation between the channel mobility and the interface state density is shown in FIG. 14.

As shown in FIG. 14, the present invention's examples 9 had a mobility higher than those of comparative examples 3 and 5 both having interface state densities higher than that of the present invention's examples 9. Also, from FIG. 14, it was found that as the interface state density is lower, the channel mobility is higher.

As described above, it was found that the mobility can be improved when the interface state density can be reduced. From this, it was found that as described above, the mobility can be effectively improved by reduction to the threshold voltage and the sub-threshold slope of the present invention.

Although the embodiments and examples of the present invention have been described thus far, it is initially expected to appropriately combine features of the embodiments and the examples. In addition, the embodiments and examples disclosed herein are illustrative and non-restrictive in any respect. The scope of the present invention is defined by the terms of the claims, rather than the embodiments described above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

INDUSTRIAL APPLICABILITY

The present invention is advantageously applied to a MOSFET in which a semiconductor layer made of SiC is formed in contact with an insulating film.

DESCRIPTION OF THE REFERENCE SIGNS

1, 3: MOSFET; 2: substrate; 10: gate electrode; 12: drain electrode; 18: back-side contact electrode; 21, 31: semiconductor layer; 23: well region; 24: source region; 25: contact region; 26: insulating film; 27: source electrode; 28: interlayer insulating film; 29: drain region; 30: MOS capacitor. 

The invention claimed is:
 1. A MOSFET comprising: a silicon carbide substrate having a main surface having an off angle of not less than 50° and not more than 65° relative to a {0001} plane; a semiconductor layer formed on said main surface of said silicon carbide substrate; an insulating film formed in contact with a surface of said semiconductor layer and having a thickness of not less than 30 nm and not more than 46 nm; and a region including a nitrogen atom for improving a channel mobility, said region at an interface between said semiconductor layer and said insulating film; the MOSFET having a threshold voltage of not more than 2.3 V, wherein the threshold voltage is a threshold voltage when an acceptor density is converted into 1×10¹⁶ cm⁻³.
 2. The MOSFET according to claim 1, wherein the MOSFET has a sub-threshold slope of not more than 0.4 V.
 3. The MOSFET according to claim 1, wherein a maximum value of nitrogen concentration is 1×10²¹ cm⁻³ or greater in said region at a portion distant away by 10 nm or smaller from an interface between said semiconductor layer and said insulating film.
 4. The MOSFET according to claim 1, wherein said semiconductor layer is formed of silicon carbide.
 5. The MOSFET according to claim 1, wherein said main surface of said silicon carbide substrate has an off orientation falling within a range of ±5° of a <11-20> direction.
 6. The MOSFET according to claim 1, wherein said main surface of said silicon carbide substrate has an off orientation falling within a range of ±5° of a <01-10> direction.
 7. The MOSFET according to claim 6, wherein said main surface of said silicon carbide substrate has a plane orientation having an off angle of not less than −3° and not more than +5° relative to a plane orientation {03-38}.
 8. The MOSFET according to claim 6, wherein said main surface of said silicon carbide substrate has an off angle of not less than −3° and not more than +5° relative to a (0-33-8) plane in the <01-10> direction.
 9. A MOSFET comprising: a silicon carbide substrate having a main surface having an off angle of not less than 50° and not more than 65° relative to a {0001} plane; a semiconductor layer formed on said main surface of said silicon carbide substrate; an insulating film formed in contact with a surface of said semiconductor layer and having a thickness of more than 46 nm and not more than 100 nm; and a region including a nitrogen atom for improving a channel mobility, said region at an interface between said semiconductor layer and said insulating film; the MOSFET having a threshold voltage of more than 2.3V and not more than 4.9 V, wherein the threshold voltage is a threshold voltage when an acceptor density is converted into 1×10¹⁶ cm⁻³.
 10. The MOSFET according to claim 9, wherein the MOSFET has a sub-threshold slope of not more than 0.4 V.
 11. The MOSFET according to claim 9, wherein a maximum value of nitrogen concentration is 1×10²¹ cm⁻³ or greater in said region at a portion distant away by 10 nm or smaller from an interface between said semiconductor layer and said insulating film.
 12. The MOSFET according to claim 9, wherein said semiconductor layer is formed of silicon carbide.
 13. The MOSFET according to claim 9, wherein said main surface of said silicon carbide substrate has an off orientation falling within a range of ±5° of a <11-20> direction.
 14. The MOSFET according to claim 9, wherein said main surface of said silicon carbide substrate has an off orientation falling within a range of ±5° of a <01-10> direction.
 15. The MOSFET according to claim 14, wherein said main surface of said silicon carbide substrate has a plane orientation having an off angle of not less than −3° and not more than +5° relative to a plane orientation {03-38}.
 16. The MOSFET according to claim 14, wherein said main surface of said silicon carbide substrate has an off angle of not less than −3° and not more than +5° relative to a (0-33-8) plane in the <01-10> direction.
 17. A method for manufacturing a MOSFET comprising the steps of: preparing a silicon carbide substrate having a main surface having an off angle of not less than 50° and not more than 65° relative to a {0001} plane; forming a semiconductor layer on said main surface of said silicon carbide substrate; and forming an insulating film in contact with a surface of said semiconductor layer and having a thickness of not less than 30 nm and not more than 46 nm, wherein the step of forming said insulating film includes the steps of: forming said insulating film by dry oxidation; and thermally treating said insulating film for improving a channel mobility using gas including a nitrogen atom as atmospheric gas; and the MOSFET having a threshold voltage of 2.3 V or smaller, wherein the threshold voltage is a threshold voltage when an acceptor density is converted into 1×10¹⁶ cm⁻³.
 18. A method for manufacturing a MOSFET comprising the steps of: preparing a silicon carbide substrate having a main surface having an off angle of not less than 50° and not more than 65° relative to a {0001} plane; forming a semiconductor layer on said main surface of said silicon carbide substrate; and forming an insulating film in contact with a surface of said semiconductor layer and having a thickness of more than 46 nm and not more than 100 nm, wherein the step of forming said insulating film includes the steps of: forming said insulating film by dry oxidation and thermally treating said insulating film for improving a channel mobility using gas including a nitrogen atom as atmospheric gas; and the MOSFET having a threshold voltage of more than 2.3 V and not more than 4.9 V, wherein the threshold voltage is a threshold voltage when an acceptor density is converted into 1×10¹⁶ cm⁻³. 